Papermaking machines generally have three zones, i.e., a forming zone, a pressing zone and a drying zone, for removing water from the web material during the papermaking process. In each of the zones, water is continuously removed from the wet paper web. Each of the zones employs a papermaking tool having a dehydrating capability.
The pressing zone employs a felt as such a papermaking tool, which runs in the warpwise direction (MD). Specifically, the pressing zone includes a pressurizing mechanism for transferring water from wet paper web to the felt to dehydrate the wet paper web while the felt and the wet paper web are traveling through the pressurizing mechanism.
The pressurizing mechanism generally comprises a pair of pressing rolls or comprises a pressing roll and a shoe shaped complementarily to a circumferential surface of the pressing roll.
The structure of the felt will be described below with reference to FIG. 1. FIG. 1 is a cross-sectional view showing a general structural arrangement of a felt 100 for papermaking according to the background art.
The felt 100 for papermaking comprises a base 20, face-side batt fibers 31 stacked on the base 20, and reverse-side batt fibers 32 stacked on the base 20. The batt fibers 31, 32 are implanted on the base 20 by needle punching or the like. The base 20 usually comprises a woven fabric formed of warp yarns 21 and weft yarns 22 by weaving.
The felt has a plurality of basic functions including a water squeezing function to squeeze water out of the wet paper web, a smoothing function to increase the smoothness of the wet paper web, and a wet paper web feeding function to feed the wet paper web.
Of these felt functions, the function to squeeze water out of the wet paper web (water squeezing function) is regarded as important. According to the function to squeeze water out of the wet paper web, water is transferred from the wet paper web to the felt under pressure while the wet paper web and the felt are traveling through between a pair of pressing rolls.
The transferred water in the felt is discharged from the reverse side of the felt under pressure or is drawn out of the felt by a suction box of the papermaking machine. Therefore, it is important that the felt should have a sustained ability to be compressed when pressurized and to be recovered when depressurized, and should also be permeable to water.
Recent trends in the papermaking technology are toward higher-speed papermaking machines for increased productivity and are toward pressing zones having rolls or shoe presses capable of higher pressurization. Therefore, the felt in the pressing zone tends to be flattened under high pressure, and to be lowering its water permeability and its ability to be compressed and recovered. As a result, the water squeezing ability of the felt is greatly reduced.
One solution to the above problems is to contain a high-polymer elastic material in a fiber layer of the felt.
For example, there is known a felt made of fibers impregnated with an emulsion resin and having a wet paper web side processed (see U.S. Pat. No. 4,500,588).
Specifically, the known felt has a batt fiber layer, on a surface of a base layer, which is impregnated with an emulsion resin. The batt fiber layer has a wet paper web side surface which is calendered into a dense and chamois-like surface. In this manner, a barrier layer is produced or a coarse fiber layer on the surface of the base layer is impregnated with an emulsion resin.
A barrier layer (nonwoven layer) is disposed on the coarse fiber layer, and a fine fiber layer is disposed on the barrier layer (nonwoven layer). The barrier layer prevents the emulsion resin from penetrating to the wet paper web side surface of the felt. As a result, the felt is prevented from becoming damp again and also from blowing for an increased papermaking rate.
The fiber layer which is contained in the high-polymer elastic material is less hydrophilic in an initial phase of use. Therefore, the felt contained in the high-polymer elastic material has a strong tendency to repel water when it is first installed in the papermaking machine.
Even if the felt is initially used while a shower of water is being sprinkled over the felt, no sufficient water penetrates the felt. Accordingly the papermaking machine needs to operate at a low rate during a certain period until the felt becomes sufficiently hydrophilic.
Patent document: U.S. Pat. No. 4,500,588
The present invention has been made in efforts to solve the above problems. It is an object of the present invention to provide a felt for papermaking which comprises batt fibers contained in and integrally combined with a high-polymer elastic material making up a batt fiber layer, which allows sufficient water to penetrate the felt from an initial phase of use immediately after the felt is installed in a papermaking machine because the high-polymer elastic material is highly hydrophilic in the initial phase of use.
Another object of the present invention is to provide a felt for papermaking which has a sustained ability to be compressed and recovered for a long period of time from an initial phase of use of the felt and which maintains a water squeezing function, a wet paper web smoothing function, and a wet paper web feeding function until the end of the period in which the felt is used.